The present invention is directed to a polyester composition resulting from a polyester blend, the polyester composition having a balance of properties more suitable for molding purposes than the polyester by itself.
There has been increased usage in recent years of thermoplastic polymers for molding useful articles. Presently there is a wide variety of articles molded from thermoplastic articles, the range extending from small articles to large articles, and from articles for low strength service to articles for high strength service.
One of the more desirable classes of thermoplastic polymers for molding articles for high strength service application is polyester. Polyesters are reasonable in cost, can be handled easily and can be molded with little difficulty.
Certain thermoplastic polyesters for molding articles for high strength service have proved to be most desirable. For instance one of the more desirable ones is poly(tetramethylene terephthalate) because this polyester exhibits a desirable overall balance of most mechanical properties such as tensile strength, flexural modulus and the like. The notched Izod impact strength, however, has been rather low and has thus been a factor limiting the use of poly(tetramethylene terephthalate) for some applications where higher impact strengths are required.
Another thermoplastic polyester is poly(1,4-cyclohexylenedimethylene terephthalate) as disclosed in U.S. Pat. No. 2,901,466. This thermoplastic polyester is an inherently brittle material, and processing of the material requires a high molding temperature (.gtoreq.290.degree. C.) which can cause degradation and increased brittleness. These factors have prevented poly(1,4-cyclohexylenedimethylene terephthalate) from generally being useful as a thermoplastic molding composition.
Still another thermoplastic polyester having similar problems of brittleness and high temperature molding requirements is a modified poly(1,4-cyclohexylenedimethylene terephthalate), which is modified with up to about 10 percent by weight of poly(oxyethylene) glycol having a molecular weight of about 500 to about 2000.
An object of the invention, therefore, is to improve the usefulness of the thermoplastic polyester, poly(tetramethylene terephthalate), as a molding plastic by substantially increasing its notched Izod impact strength at room temperature and its unnotched Izod impact strength at low temperature, with only some loss of tensile strengths and flexural properties, although these latter properties are still at a desired acceptable level for molding purposes.
Another object of the invention is to make the use of the thermoplastic polyesters, poly(1,4-cyclohexylenedimethylene terephthalate) and the above-described modified poly(1,4-cyclohexylenedimethylene terephthalate) more attractive as molding plastics by increasing their notched Izod impact strengths and by substantially increasing their unnotched Izod impact strengths with only some loss in tensile strengths and flexural properties, although still also being at a desired acceptable level for molding purposes; and to lower the molding temperature requirements by about 20.degree. C. to 30.degree. C. at the same time.
Other objects inherent in the nature of the disclosed invention will become apparent to those skilled in the art to which this invention pertains.
For purposes of the description of this invention, the notched Izod impact test (ASTM D-256) indicates the energy required to break a notched specimen. The specimen, usually 1/8 inch by 1/2 inch by 2 inches, is clamped in the base of a pendulum testing machine so that it is cantilevered upward, with the notch facing the direction of impact. The notch has a radius of 0.010 inch. The pendulum is released and the force consumed in breaking the specimen is calculated from the height the pendulum reaches on the follow-through. The Izod impact test is calculated as foot-pounds per inch of notch. The Izod value is useful in comparing various types or grades of a plastic. It may indicate the need for avoiding sharp corners in parts made of materials that may prove to be notch sensitive.
The unnotched Izod impact test is the same as described above, with the exception that the specimen is not notched.
Also, for purposes of the description of this invention, the flexural modulus test is described in ASTM D-747; the tensile strength test in ASTM D-638; and the heat deflection temperature (HDT) in ASTM D-648.